Accumulation of purulent, infected sputum in the airways of patients with cystic fibrosis(CF) obstructs airways and causes chronic, symptomatic infection(1). Sputum in CF is rich in leukocyte-derived extracellular DNA that greatly contributes to the abnormal viscoelasticity of sputum (2,3). Recombinant human DNase I (rhDNase), a copy of the native human enzyme responsible for hydolyzing extracellular DNA, dramatically alters the viscoelastic properties of CF sputum in vitro (4). Aerosol administration of rhDNase with a pneumatically driven hand-held nebulizer improves pulmonary function in CF patients within days of initiating therapy (5,6). Longer-term administration of rhDNase also reduces the need for parenteral antibiotic treatment of respiratory tract infections(7). The purpose of this investigation is to evaluate the usefulness of rhDNase in children who are younger than 5 years of age.